feat(frontend): support fragment level progress for snapshot backfill

[kwannoel]

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What's changed and what's your intention?

This PR improves the backfill progress tracking for snapshot backfill by:

1. Restructuring the backfill progress test files for better organization
2. Adding support for tracking progress in snapshot backfill mode by:
   • Tracking the minimum epoch in backfill progress
   • Enhancing the rw_fragment_backfill_progress view to show 100% completion when the minimum epoch exceeds the backfill epoch
   • Adding is_snapshot_backfill and backfill_epoch fields to the table scan information
3. Extending the test cases to verify both regular and snapshot backfill progress

These changes provide more accurate progress reporting for snapshot backfill operations, showing 100% completion when the backfill has processed all data up to the snapshot epoch.

Checklist

• I have written necessary rustdoc comments.

[kwannoel]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(frontend): support fragment level progress for snapshot backfill #22166 👈 (View in Graphite)
• feat(catalog): add fragment backfill progress view #22124
• feat(catalog): add rw_table_scan system catalog #22123
• feat(frontend): support querying internal backfill table progress #22122
• main

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